The present invention relates to the radiography arts. It finds particular application in conjunction with switch mode, or inverter x-ray generators and will be described with particular reference thereto. However, it is to be appreciated that the invention will also find applicability in radiographic and other analogous systems with significant ripple.
A shadowgraphic x-ray system commonly includes an x-ray generating tube which projects radiation through a patient receiving region to a sheet of x-ray film or other radiation detecting medium. The x-ray tube includes a power supply which provides a voltage across anode and cathode of the x-ray tube in kilovolts (kV) and a filament current. The tube anode current (mA) is controlled by the filament current. Both the tube voltage or kV and the anode current or mA are selectively adjustable. A timer times the selectable duration of each exposure, normally measured in seconds (s).
The contrast of the x-ray film image is controlled primarily by the kV peak. The density of the exposed film is determined by the x-ray dose or exposure which is commonly designated by the product of the anode current mA and times. This product is commonly denoted as the mAs value. I n operation, the operator commonly sets the kV value such that the resultant image has a selected contrast. In a full manual operation, the operator commonly sets either the mAs value, or the anode current, and the exposure time in order to expose the film to a desired film density. In theory, the resultant film density, for a given kV value, should be the same for a selected mAs value regardless of whether a longer exposure time and a lower anode current or shorter exposure time and a higher anode current are selected.
In conventional three phase radiographic equipment, especially those having twelve pulse rectification of the output voltage, the film density varies little, if at all, with exposure mA for the selected mAs value. However, in radiographic equipment with less than twelve pulse rectification, which includes conventional three phase six pulse equipment, conventional single phase two pulse equipment, and switchmode or inverter generators, the film density or x-ray dose is typically not uniform over the range of times and anode currents Shorter duration, higher current exposures tend to have a lower overall dose, hence, a lower film density relative to the images taken with a lower anode current, hence longer duration exposure.
It should be noted that one reason for the reduced dose at higher anode currents in radiographic equipment with less than twelve pulse rectification is that the kV ripple generally increases at higher tube currents for such equipment. While the correct kV peak value, hence the correct image contrast, is achieved at both low and high anode current, the density of images of the same mAs will be lower at higher anode current settings due in part to the higher kV ripple.
In an auto exposure mode, the operator typically sets the desired contrast, or kV value and selects the tube current. The control circuit then integrates the dose of radiation actually received by a portion of the film. When the dose corresponding to the desired film density is reached, the automatic exposure control terminates the exposure. Although the automatic exposure mode produces images of the selected density, at higher tube currents the exposure time is longer than predicted.